Advances in drug abuse prevention have led to the development of effective research-based prevention programs that have been shown in rigorous evaluation studies to prevent the onset and escalation of alcohol, tobacco, and other drugs during adolescence. Dissemination initiatives by NIDA and other federal agencies have gradually increased the adoption and implementation of evidence-based programs. Yet, relatively little is known about how to best surmount the many barriers to effective implementation, appropriate adaptation, and maintenance. This REVISED application proposes a 5-year multi-site prevention field trial to address critical issues related to implementation fidelity, adaptation, and institutionalization. The study will consist of a planning/developmental phase (one year), an implementation phase (three years), and a data analysis phase (one year). The proposed research is designed to identify effective ways to enhance implementation fidelity of evidence-based prevention programs designed for middle/junior high schools, identify factors associated with implementation fidelity, determine the extent to which enhanced fidelity increases student drug-related outcomes, and examine the relationship between fidelity, adaptation, and institutionalization. The prevention program selected for use in this study is the Life Skills Training (LST) program, a rigorously tested school-based prevention program identified as an exemplary or model program by multiple federal agencies and currently the most widely disseminated evidence-based program for middle school students. Schools (N = 44) from participating sites will be randomly assigned to one of two conditions: (1) LST and standard provider training or (2) LST and provider training plus a newly developed fidelity enhancement (FE) intervention designed to be flexible and feasible. Data will be collected from students (N = 4,000) and teachers (N=120) and project staff will observe program implementation to assess fidelity using measures of exposure, adherence, and quality. An array of multivariate analyses will be conducted to address the specific aims of the study and answer the key research questions. The study will be guided by an experienced team of prevention scientists, internationally known consultants, and a community practitioner advisory board that will participate in all phases of the study.